This invention relates to core constructions for nuclear reactors, and more particularly it is concerned with a core construction for a boiling-water reactor.
Generally, the core of a boiling-water reactor is composed of a multiplicity of fuel cells each including four fuel assemblies arranged in adjacent relationship and one control rod insertable into a position surrounded by such four fuel assemblies. In a boiling-water reactor of the prior art having such core construction, burning of the nuclear fuel causes variations to occur in the amount of fissionable material produced and the distribution of the amount of such fissionable material during normal power operation of the reactor, and consequently the core shows changes in excess reactivity and changes in power distribution. To offset or adjust such core excess reactivity changes, i.e., core reactivity changes, and such power distribution changes during normal power operation of the reactor, it has hitherto been common practice to, utilizing a plurality of control rods, alter the selection of control rods to be inserted and/or the number of inserted control rods thereby to change the control rod pattern, and/or vary the depths of insertion of control rods without changing the pattern. To this end, it has hitherto been necessary to operate the control rods several times during one operation cycle lasting for one year, for example, to vary the control rod pattern and/or the depths of insertion of the control rods. However, in a core of the prior art, it has been considered necessary to operate the control rods after lowering the power level of the fuel adjacent the control rods below a predetermined level, to ensure that the fuel is maintained in a sound condition. Thus, the present practice followed in varying the control rod pattern and the depths of insertion of the control rods consists in temporarily lowering the reactor power for operating the control rods and then restoring the reactor power to its original level following completion of operation of the control rods. Because of the need to follow such practice, a boiling-water reactor of the prior art has had the disadvantages that its operation procedures are complex and that the rate of utilization of a plant having such reactor is low.
Also, in a boiling-water reactor of the prior art, flattening of the radial power distribution has been unobtainable at the terminating stage of an operation cycle at which the control rods are fully withdrawn, with a result that the center zone of the core has shown a tendency to becoming high in power. This has made it necessary to effect shuffling of the fuel assemblies when fuel replacements are effected upon termination of an operation cycle. This has also been a factor concerned in rendering the operation procedures of a conventional boiling-water reactor complex and lowering the rate of utilization of a plant having such reactor.